An example of backlight devices used in liquid crystal display devices is known as an edge-light type backlight device. Such an edge-light type backlight device may include a light guide plate disposed in a case for directing light emitted from a light source toward a display surface. In general, a base material for a light guide plate made of resin is processed by blanking with a frame-shaped die and thus a primary molding member is obtained.
Patent document 1 discloses a method of forming a light guide plate. In this method, a resin plate is extruded to form a primary molding plate for a light guide plate, and a stamper is pressed against a side surface of the primary molding plate. The stamper is a portion of a heated plate and includes a pattern or a flat surface. Therefore, the side surface of the primary molding plate is heated and pressed by the stamper to have the pattern thereon or to be a flat surface.